Intelligence and Miscellaneous Articles. 317 



low temperature of the gases which form it, acts on light as energeti- 

 cally as, though in a different manner from the sun's atmosphere. The 

 earth's atmosphere produces in the red, orauge, and yellow of the 

 spectrum a system of lines ten times as numerous^as the solar lines 

 of these regions. On the contrary, in the green, the blue, and the 

 violet, the lines of solar origin predominate. Thus these two atmo- 

 spheres, so different in their own temperatures, are not less so in 

 their actions upon light. In a certain sense they divide the spec- 

 trum : the atmosphere of the earth, an atmosphere at a low tempe- 

 rature, acts specifically on the rays of great wave-length ; the solar 

 atmosphere, an atmosphere with a high temperature, exerts its rela- 

 tive action upon rays of short wave-length. This subject will have 

 to be reverted to. 



The action of our atmosphere being demonstrated, it remained to 

 inquire to what elements of this atmosphere this action must be at- 

 tributed. 



An attentive study of the solar spectrum had led me, a couple of 

 years ago, to attribute to the aqueous vapour dissolved in our atmo- 

 sphere a very important, if not an entire part in the production of 

 the telluric lines of the solar spectrum*. 



In fact, comparisons, followed for a long time, on the solar light 

 during various seasons of the year showed very clearly that for the 

 same heights of the sun certain lines of the spectrum of this star were 

 more pronounced as the dew-point was higher. 



Observations which I have made on the Faulhorn further confirm 

 this view ; for on extremely dry days I have seen the lines in ques- 

 tion disappear almost entirely from the spectrum. 



Thus, in the experiment on the Lake of Geneva, I have been led 

 to choose this place as the basis of experiments by the consideration 

 that the luminous pencil, in grazing the surface of the water, must 

 traverse layers of air which are necessarily more moist, which would 

 add to the chances of success ; and the result has confirmed this 

 notion. 



There could thus be little doubt as to the action of aqueous va- 

 pour ; yet it was necessary, seeing the importance of the result, to 

 submit this point of theory to a direct verification, by investigating 

 the modifications which a well-defined beam of light of known com- 

 position experienced by the fact of its passage through a tube of suf- 

 ficient length containing only aqueous vapour. 



This experiment presented, unfortunately, great practical difficul- 

 ties. Our atmosphere contains such a quantity of aqueous vapour, 

 that to realize artificially the effects it produced on solar light, I was 

 led to the use of apparatus of exaggerated dimensions and difficult 

 to manipulate. 



A first trial was made at the central workshop for lighthouses t. 

 M. Allard, chief engineer at this establishment, was so good as to 



* See, in reference to this, the discussion which has arisen between 

 Father Secchi and myself, Comptes Rendus, July 12, 1863; July 27, 1863; 

 July 25, 1864 ; January 30, 1866. 



t Comptes Rendus, January 30, 1865. 



